The sei whale (Balaenoptera borealis) is one of the least known
whale species. Information on sei whale distributions and its regional
variability in the south-eastern Pacific Ocean are even more scarce than that
from other areas. Vocalizations of sei whales from this region are not
described yet. This research presents the first characterization of sei whale
sounds recorded in Chile during the austral autumn of 2016 and 2017.
Recordings were done opportunistically. A total of 41 calls were identified
to be sei whale downsweeps. In 2016, calls ranged from an average maximum
frequency of 105.3 Hz down to an average minimum of 35.6 Hz over 1.6 s
with a peak frequency of 65.4 Hz. During 2017, calls ranged from an average
maximum frequency of 93.3 down to 42.2 Hz (over 1.6 s) with a peak
frequency of 68.3 Hz. The absolute minimum frequency recorded was 30 Hz and
the absolute maximum frequency was 129.4 Hz. Calls generally occurred in
pairs, but triplets or singles were also registered. These low-frequency
sounds share characteristics with recordings of sei whales near the Hawai'ian
Islands but with differences in the maximum frequencies and duration. These
calls distinctly differ from sounds previously described for sei whales in
the Southern Ocean and are the first documented sei whale calls in the
south-eastern Pacific.

The sei whale (Balaenoptera borealis; Lesson 1828) is the third largest rorqual in the
Balaenopteridae family, after the blue whale (B. musculus) and the fin whale (B. physalus). It is
also one of the least known whales. The sei whale is a cosmopolitan species
found in temperate oceans and subpolar areas (Mackintosh, 1942; Gambell,
1968; Rice, 1998; Horwood, 2002; Reeves et al., 2002; Jefferson et al., 2008). It
prefers deep offshore waters with temperatures below 20 ∘C and
avoids semi-enclosed bodies of water (Omura and Nemoto, 1955; Gambell,
1985). North Atlantic, North Pacific, and Antarctic populations are almost
certainly separated and probably subdivided into geographic stocks (Horwood,
1987; Baker et al., 2004; Kanda et al., 2006; Huijser et al., 2018). The International Whaling
Commission in 1991 divided the global sei whale population into “stocks”
(based on the distribution of catches, sightings, and mark–recapture data)
for management purposes (Donovan, 1991). However, genetic studies provide a
different population distribution. For example, in the North Atlantic Ocean,
the sei whale population from Iceland, the Gulf of Maine, and the Azores
shares the same genetic diversity, showing the wide latitudinal and
longitudinal ranges in which they moved. Furthermore, the genetic divergence between
North Pacific and North Atlantic stocks is well known, but no studies of
this genetic structure between hemispheres or within the Southern Ocean have
been presented (Huijser et al., 2018). In the Southern Hemisphere, sei whale
sightings have been recorded from the Subtropical Convergence to the Antarctic
Convergence, but the only observation records of adult animals come from the south of the Antarctic
Convergence in austral summer (Gambell, 1974;
Lockyer, 1977). In general, sei whales migrate seasonally from the
reproduction areas in low latitudes in winter to their feedings areas in
high latitudes in summer (Reeves et al., 1998). Reproduction areas are poorly
known (Perry et al., 1999) and feeding areas show great variability between years
(Jonsgård and Darling, 1977). Population boundaries and migratory
patterns are also poorly understood. In the austral summer there are
concentrations of sei whales between 40 and 50∘ S;
older, larger individuals tend to travel to the northern Antarctic, while
smaller, younger individuals tend to stay at lower latitudes (Rice, 1998;
Acevedo et al., 2017).

Because of their smaller size, speed, and elusiveness, sei whales were
comparatively less important as target species for hunting until the early
1960s. After the decline in the most profitable species such as blue whales,
fin whales, and humpback whales, the whaling industry increased the hunting
pressure on sei whales (Gambell, 1985). Thirty years ago, between the
Antarctic and the North Pacific, many whales were taken from the coasts of
Peru and Chile (Tonnessen and Johnsen, 1982). Most captures were carried out
by pelagic whaling in the Antarctic, which hunted more than 110 000 sei
whales between 1960 and 1970 (Horwood, 2002). The International Whaling
Commission estimated the size of the sei whale populations in the South
Hemisphere to be 37 000 individuals after the cessation of commercial
captures in 1984, while this number was estimated to be 191 000 in the 1940s
(Gambell, 1985). Between 1929 and 1983, sei whale captures represented
17.3 % of the total catch of whales in Chile. It was the third most hunted
species, with approximately 1664 individuals captured principally on the
north and central coasts (Aguayo-Lobo, 1974; Aguayo-Lobo et al., 1998), although
these include an unknown number of Bryde's whales (Valdivia et al., 1981; Gallardo
et al., 1983; Aguayo-Lobo et al., 1998). After the whale-hunting moratorium imposed by
the International Whaling Commission in 1980, several research projects
focused on the populations and recovery status of large whales such as
right whales, humpback whales, blue whale, and fin whales (Reeves et al., 2002).
Since 1976, sei whales have been listed as endangered (IUCN, 2018). Today,
sei whales are the least studied of the large whales, and there has been a
lack of data since the end of the commercial hunting (Prieto et al., 2011).

In Chile, there are opportunistic sightings and strandings of sei whales
from Antofagasta (in the north) to the Magellan Straight (in the south),
including the islands of Juan Fernandez (Gallardo et al., 1983; Schlatter, 1987;
Aguayo-Lobo et al., 1998; Findlay et al., 1998; Pastene and Shimada, 1999; Guzmán,
2006; Acevedo et al., 2017). Many sightings in Central Chile and northern
Patagonia (33–48∘ S) have been reported since 1966,
when 286 whales were sighted in March of 1966 (between 43 and
45∘ S) and 114 in October of the same year (between 46 and
48∘ S), all between 30 and 190 km off the shore (Aguayo-Lobo,
1974). In March 1968 Japanese whalers reported the sightings of several
hundreds of sei whales between 46∘ 40′ and 48∘ S, with a
peak concentration 30 km off the coast of the Tres Montes Peninsula at the
northern limit of the Penas Gulf (Pastene and Schimada, 1999). In 2015, at
Penas Gulf the largest recorded baleen whale mass mortality event was
reported, with 363 registered carcasses of baleen whales (Häussermann et al.,
2017). Genetic and morphological analysis confirmed that the examined
animals were sei whales (Häussermann et al., 2017). These historical
sightings support the hypothesis of Guzmán (2006) about the presence of sei
whales feeding in Chilean Patagonia between Chiloé Island and the Magellan
Strait (Acevedo et al., 2017).

Since the sei whale is endangered and poorly known, population studies are
crucial as a support for its conservation. Autonomous passive acoustic
monitoring devices facilitate the monitoring of cetaceans by recording their
vocal signals. Passive acoustic data can then be used to characterize and
understand the whales' acoustic behaviour and determine their distribution patterns
in time and space (Clark and Ellison, 1989; Richardson et al., 1995).

Acoustic signals produced by sei whales are poorly known (Prieto et al., 2011).
To date, vocalizations have been described from six different geographic
areas: New England (USA), Nova Scotia (Canada), Hawai'i (USA), Azores
(Portugal), Auckland Islands (New Zealand), and the Antarctic Peninsula
(Thompson et al., 1979; McDonald et al., 2005; Rankin and Barlow, 2007; Baumgartner
et al., 2008; Calderan et al., 2014; Romagosa et al., 2015). There is no record of sei
whale vocalizations from the south-eastern Pacific Ocean. Comparison of intraspecific sounds from different geographic regions is interesting for
possible acoustic clues to both stock and taxonomic identities.
Consequently, the aim of this work is to describe sei whale vocalizations
based on opportunistic recordings at the Penas Gulf, Chile, and to obtain a
framework baseline regarding the characterizes of sei whales populations in the
south-eastern Pacific Ocean.

Two cruises to the Penas and Tres Montes gulfs (46.2–48.0∘ S,
74.0–75.4∘ W) aboard the motor sailing vessel Saoirse were carried
out in May 2016 and May 2017, during which biological, oceanographic, and
acoustics studies were carried out (Fig. 1). Marine mammals were identified
visually with binoculars and the naked eye by a team of experienced marine
mammal observers in the vessel.

Figure 1Study area including sighting and recording locations. The whale
tail indicates the area where sei whale were sighted and vocalizations were
identified.

Two different hydrophones were used for the recordings: an icListenHF
hydrophone (sensitivity −171 dBV re 1 µPa with pre-amp; frequency
response 10–200 kHz from Ocean Sonic, Canada) and a SoundTrap 202 STD
hydrophone (sensitivity −205 dBV re 1 µPa; frequency response 60 000 Hz
±3 dB from Ocean Instruments, New Zealand). Also, we made stereo
recordings on several occasions with an HTI-96-MIN hydrophone (flat
frequency response from 0.02 to 30 kHz) connected to a handy recorder
(H4nPro from ZOOM).

Opportunistic and planned recordings were carried out depending on the
weather conditions and the vessel location. In 2016, recordings were carried out for no longer than 1 day at each location where hydrophones were suspended over the
side of the vessel. However, in 2017, at three different locations,
hydrophones were also deployed between 2 and 5 days. In these cases, hydrophones were
deployed at a depth of 5 or 10 m on rocky bottom with about 40 m bottom
depth using a row with a mooring at the end and surface buoys on the
opposite side. In both years, during the day or night, hydrophones were
deployed at a depth of 5 and 10 m from the stationary vessel.
Opportunistic recordings were continuous, but at night when the vessel
was at anchor, the recordings were at intervals of 10–30 min each hour
to guarantee the ability of the acoustic personnel to study the recording the next day.

During all the recordings, the engine vessel was turned off. All the
recordings were stored in the internal card memory of the equipment, and at
the end of the day these were downloaded onto a portable computer.

Audio data were analysed using the Raven Pro 1.5 (Cornell University, Ithaca,
NY) software. Low and high frequency (Hz), frequency range (Hz), peak
frequencies (the frequency at which the maximum power occurred within a
call), and duration (s) for all calls found and attributed to sei whales were
analysed from spectrograms and waveform plots created in Raven Pro 1.5 (Hann
window; 50 % overlap; window size 14 563 samples; DFT 16 384 samples).

Sei whales were sighted on both expeditions (2016 and 2017). In addition,
during one day in 2017, humpback whales were sighted. Sound was recorded
on 16 days in 2016 at 23 opportunistic recording locations and on
19 days at 24 locations in 2017. A total of 363 h was recorded between
both expeditions; because the recordings were not continuous every
day, this number is made up of 136 h (63 archives) in 2016 and 227 h (483 archives) in 2017.
Recordings were not of the same duration or the same sampling rate because they
were opportunistic. Sei whale calls were found in eight archives for 3 days (on
7 May 2016 and 10–11 May 2017) at two different locations (one in 2016 and
other in 2017) (Table 1). In acoustic data from 2016, sei whale calls were
detected when sei whales were sighted closer to the vessel (Fig. 2). In 2017,
between 8 and 10 May, sei whales were sighted in the area after sei whale
calls had been recorded (Fig. 2). Sei whale calls from 2016 were only
recorded around midday, while in 2017 they were recorded in the late
afternoon or at night (Table 1).

Only on two occasions were humpback whale sounds above 2000 Hz detected: in
the rest of the analysed audios, only sei whale sounds were detected without
associated calls. Only calls with high visual quality were measured. All
vocalizations reported in this study were identified as downsweep calls
(Fig. 3). We identified a total of 41 calls: 5 calls in 2016 and 36 in 2017.
In 2016, calls ranged from an average maximum frequency of 105.3 Hz
(SD = 18.3 Hz) down to an average minimum frequency of 35.6 Hz
(SD = 4.6 Hz) over 1.6 s (SD = 0.1 s) with a peak frequency of
65.4 Hz (SD = 14.1 Hz) (Table 2). In 2017, calls ranged from an average
maximum frequency of 93.3 Hz (SD = 10.9 Hz) down to 42.2 Hz (SD = 5.6 Hz)
over 1.6 s (SD = 0.3 s) with a peak frequency of 68.3 Hz (SD = 14.2 Hz).
The minimum frequency was 30 Hz and the maximum frequency was 129.4 Hz.
Calls occurred in pairs (n=12), singles (n=5), or triplets (n=4) (Table 2).

Figure 3Spectrogram of sei whale vocalization recorded with the
hydrophone (32768 FFT, Hamming window). (a) Five-second spectrogram zoomed
in on a pair of calls. (b) Five-second spectrogram of a pair of call. (c) A
pair and a single sei whale call within 15 s.

Table 2Comparison of the frequency and timing of recorded calls of sei
whales in the present study with studies in other areas. Values are mean
value ± standard deviation. ND: no data (the study did not include
that information).

Given that recordings from this project were opportunistic, and without
digital acoustic recording tags (DTAGs) deployed in sei whales, we cannot prove
the origin of the calls. However, we can confirm with reasonable certainty
that vocalizations recorded off Penas and Tres Montes gulfs were produced by
sei whales, due to the sightings of this species during the recordings and
the expeditions. Blue whales (Balaenoptera musculus), fin whale
(Balaenoptera physalus), or minke whales (Balaenoptera acutorostrata) produce downsweep as well (Thompson et al., 1996; Schevill
and Watkins, 1972; Watkins, 1981). Bryde's whales (Balaenoptera brydei) also have several call types, including downsweep, but they inhabit
tropical and subtropical waters, and we do not have any record in this area
yet (Omura, 1959; Wade and Gerrodette, 1993; Oleson et al., 2003).
Generally, fin whales' downsweep has initial frequencies below 35 Hz and
final frequencies around 20–18 Hz (Watkins, 1981), similar to minke
whales but with shorter durations (0.2–0.3 s) and higher frequencies
(130–60 Hz) (Schevill and Watkins, 1972). Minke whales in the North
Atlantic produce long pulse trains (Mellinger et al., 2000). These were not
recorded in this area. Fin and minke whale downsweep is definitively
different from our recordings. Only downsweeps from blue whales described in
Chile, through the DTAG data, has a lower peak frequency and duration; the
minimum frequency of blue whales is higher than that in our results, and
downsweep had been accompanied by the south-east Pacific signal type 2 (SEP2)
(Saddler et al., 2017), supporting our results that our records are really
from sei whales. In addition, the sei whale vocalizations described here show
very similar characteristics to those described off the Azores by
Romagosa et al. (2015), off New England by Baumgartner et al. (2008), and off
Hawai'i by Rankin and Barlow (2007). In these areas, sei whale vocalizations
are characterized by low-frequency downsweeps. However, sei whale sweeps
recorded off Nova Scotia by Thompson (1979) and Knowlton et al. (1991) or in
Antarctic waters by McDonald (2005) and Gedamke and Robinson (2010) are
different from our recording and are characterized by higher frequencies.

Rankin and Barlow (2007) describe two ranges for the low-frequency
downsweeps (100–44 and 39–21 Hz) with durations of 1.0 and 1.3 s,
respectively. In the present study, the minimum frequency was 30 Hz, being
the average calls in the upper range defined by those authors. The range of
frequencies described here is similar to what Baumgartner et al. (2008),
Newhall et al. (2012), and Romagosa et al. (2015) reported, although the
maximum frequency reported in the present study is higher. The higher-frequency calls recorded in the North Pacific (Hawai'i) and in the present
study are similar, but our results showed higher frequencies in the top range
(maximum of 111.4 vs. 129.4 Hz) and a longer duration
(maximum 1.27 vs. 2.27 s). The similarities could be expected
due to the possibility of there being a stereotypical call used in feeding
grounds, as suggested by Romagosa et al. (2015). However, sei whales recorded
in this study have shown a different call with higher frequencies and longer
durations than those detected from North Atlantic or North Pacific waters.

In the sub-Antarctic Auckland Islands, a series of four calls is predominant
(Calderan et al., 2014), but the calls recorded at Penas Gulf occurred
principally in pairs, although single calls and triplets were also detected.
During this study, no four-call series were recorded as has been done in the North Atlantic or Pacific waters (Baumgartner et al., 2008; Newhall et al.,
2012; Romagosa et al., 2015).

Sei whale calls from Antarctic waters are characterized by broadband, tonal,
frequency-modulated vocalizations between 100 and 600 Hz with durations
between 1 and 3 s (McDonald, 2005; Gedamke and Robinson, 2010). These calls
do not present similarities with the calls recorded here. This may be due a
geographic separation of the populations, suggesting that different sei whale
populations produce different stereotypical calls. The structure of the calls
of sei whales is more variable between different whales than amongst the
calls by an
individual whale (Baumgartner et al., 2008). This suggests that sei whales
present in Antarctic waters do not transit through southern Chile, or at
least near the shore of Patagonia, in their migration to the breeding grounds
in lower latitudes. Thus, the sei whales found in Chile, near the shore in
Penas and Tres Montes gulfs, might represent a different population. As
recordings were opportunistic, the duration of recordings was variable, and
this maybe influenced the few calls that we recorded, so visual observations,
tagging efforts, and genetic studies are needed to verify this hypothesis.

In the present study, most acoustic activity was recorded during the night,
while Baumgartner and Fratantoni (2008), Newhall et al. (2012), and Romagosa
et al. (2015) recorded calls mostly during the day. These darkness patterns
coincide with results from humpback whale songs reported from Chile
(Español-Jiménez and van der Schaar, 2018). However, low-frequency
sei whale downsweeps may have a different function from the stereotyped
humpback vocalizations considered as songs (Edds-Walton, 1997). Though the
behaviour of sei whales is poorly studied, most studies on this species state
that sei whales prefer offshore waters, but these new records and sightings
along the coast of Penas and Tres Montes gulfs (Aguayo-Lobo, 1974; Pastene and
Schimada, 1999; Häussermann et al., 2017) demonstrated a wide habitat
range, with the whales probably following productive feeding areas. If this
is true, it is reasonable to assume that the calls of sei whale are
influenced by the feeding conditions (as proposed by Baumgartner and
Fratantoni, 2008) and have communicative functions, e.g. cooperatively
searching for prey as suggested Newhall et al. (2012). Baumgartner and
Fratantoni (2008) hypothesize that calling rates are reduced at night while
the whales are feeding but increase with social activity during the day when
copepods are either more difficult or less efficient to capture at depth. Our
data could not support this hypothesis since calls were recorded at night, and
it was not possible to observe what activities the whales were engaged in.
Another factor that could be important in the discussion about the acoustic
behaviour is background noise, which might be masking biologically important
signals and impede the communications between individuals (Clark et al.,
2009).

This new description of sei whale calls adds knowledge to the vocalizations
and distribution of an endangered species (IUCN, 2018) red-listed under
criteria A-1. It is also listed in Appendix I (“Endangered migratory
species”) and II (“Migratory species with unfavorable conservation status
which require international agreements for their conservation and
management”) of the Convention on the Conservation of Migratory Species of Wild Animals (Bonn Convention, 1979). Satellite tracking of the
Chilean sei whale population, individual photo identification, distribution,
and characteristics of the prey species, behavioural, genetic, and
oceanographic studies are necessary to test some hypotheses and improve our
understanding of this species.

This research would not have been possible without the assistance of
Keri-Lee Pashuk and Greg Landreth of Patagonia Projects who organized the
funding and operations of expedition vessel SRV Saoirse supporting the HF29 and
HF32 expeditions; in addition, we want to thank Michael Kean, Gastón
Herrera, Sebastián Durán, Cristian Santana, Valentina Molinos; Mark
Woods of Ocean Sonic and Daniel Zitterbart for providing hydrophones; and John
Atkins at Ocean Instruments for help in the configurations. These
expeditions were funded by the Blue Marine Foundation, Global Marine Networks
(USA); Iridium Communications Inc.; and Deep Trekker Inc (Canada), MERI
Foundation, Huinay Foundation and Fondecyt project no. 1161699. We are very
grateful to Alessandro Bocconcelli and Joe Warren for their feedback on this
research, also to the anonymous referee. This is publication no. 161 of
Huinay Scientific Field Station. Paulina Bahamonde is supported by Nucleo
Milenio INVASAL funded by Chile's government program, Iniciativa Cientifica
Milenio from Ministerio de Economia, Fomento y Turismo.

Omura, H. and Nemoto, T.: Sei whales in the adjacent waters of Japan, III,
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Whaling Commission, Special Issue, 10, 79–87, 1955.

Omura, H.: Bryde's whale from the coast of Japan, The Scientific Reports of
the Whales Research institute, 14, 1–33, 1959.

Perry, S. L., DeMaster, D. P., and Silber, G. K.: The great whales: history
and status of six species listed as endangered under the U.S. Endangered
Species Act of 1973, Mar. Fish. Rev., 61, 1–74, 1999.

Sei whales are one of the least known baleen whales. We found them in southern Chile, on the Patagonian coast. We set up hydrophones in the Penas Gulf in 2016 and 2017 to investigate how this Patagonian sei whale might be communicating. We could identify sei whale downs-weep calls (a type of vocalization that starts at a high frequency and ends at a lower). We found that sei whales in the Penas Gulf perform calls distinctly differently from the sounds previously described.

Sei whales are one of the least known baleen whales. We found them in southern Chile, on the...